梯形面积=(上底+下底)x高x1/2
=(a+2a+3)x(1.5a+1.5)x1/2
=[(3a+3)(1.5a+1.5)]/2
=(4.5a²+4.5a+4.5a+4.5)/2
=4.5a²+9a+4.5/2